Anadromous fish - Sokagyo (English spelling)

This refers to fish that feed and grow in the ocean, then migrate to rivers or lakes to spawn. They are also called anadromous fish, and are used in contrast to anadromous fish such as eels that travel down to the sea to spawn. Many lay large, sticky eggs, and some individuals are landlocked in rivers or lakes. Smelt, Shishamo, and icefish live in coastal areas, and in the spring they travel up rivers to spawn at the mouths of rivers. Three-spined sticklebacks that enter the sea spend the winter on the coast, then travel upriver in March to spawn downstream at an altitude of less than 100 meters. Some types of Japanese dace also live in inland bays, and spawn in rivers in the spring.

Lampreys, sturgeons, salmon and trout live in the sea for one to several years, grow large, and then return to rivers to spawn in the upper and middle reaches of the river. Salmon and trout in particular have the habit of returning to their natal rivers, a practice known as natal river homing. Even within the same species, different species have different seasons for migrating upstream, as well as differences in body size and morphology, and these traits are passed on to their offspring. It is interesting to note that these fish species, which are lineage-distant, all have in common that they each spawn at different times of the year, which has the effect of spreading out damage from floods, other natural disasters, and disease. Furthermore, differences in the peak fishing seasons between species lead to stable fish prices.

[Akira Ochiai and Kunio Amano]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

海洋で餌(えさ)をとって成長し、産卵のために河川または湖へ回遊する魚類をいう。遡上魚(そじょうぎょ)ともいい、ウナギのように産卵のために海へ下る降河型の魚と対比して用いる。多くは、大きくて粘着性の卵を産み、河川または湖沼に陸封される個体をもつ。ワカサギ、シシャモ、シラウオなどは沿岸域にいて、春に川をさかのぼって河口域で産卵する。海に入ったイトヨは沿岸で越冬したのち、3月ごろに遡上して標高100メートル以下の下流域で産卵する。ウグイも内湾にすみ、春に川で産卵する型がある。

　ヤツメウナギ類、チョウザメ類、サケ・マス類は、海で1年から数年間生活して大きく成長し、川へ帰って上・中流域で産卵する。とくにサケ・マス類は母川へ帰る習性があり、これを母川回帰という。同じ種類でも、種族によって川をさかのぼる季節や、体の大きさ、形態にも違いがあり、それらの形質が子孫に遺伝される。系統的にかけ離れたこれらの魚類間に、共通して、それぞれ産卵する季節をずらした種族があることは興味深く、洪水その他の天災、病気などによる被害を分散させる効果がある。また、種族間での漁期の最盛期の差は魚価の安定につながる。

［落合　明・尼岡邦夫］

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